The present invention relates to hot melt adhesives, and more particularly to a heat stable, color stable hot melt adhesive useful for product assembly applications such as a multi-line adhesive used in the construction of disposable diapers, sanitary napkins and bed pads.
While a wide range of uses for hot melt adhesive compositions are known throughout the disposable article industry, it has been found that a hot melt adhesive used for bonding in a particular use or application may be completely unsuitable for other uses or applications. Thus, various hot melt adhesive compositions have been proposed for use in the construction of disposable articles. Depending upon the type of construction employed, the adhesive must possess certain physical properties. Perhaps the most stringent properties are those required of adhesives to be used in the bonding of polyethylene films, or the like, to tissue or non-woven substrates in the production of articles, particularly diapers, sanitary napkins and bed pads, using multi-line construction techniques. This class of disposable construction presents unique problems for the adhesive formulator. The adhesive must possess a high degree of adhesion since it is applied in the form of a number of very fine parallel longitudinal stripes thus requiring each line of adhesive to possess exceptionally high bonding properties. The adhesive must also possess sufficient adhesive and cohesive strength to provide high bond strength values when subjected to stress so the constructions cannot be easily separated. As an additional criterion, it is necessary that the adhesive, upon application, not absorb throughout the actual disposable construction and that the adhesive bonds not only remain secure but also be flexible even after prolonged periods of storage.
Though an adhesive composition may satisfy all the above criteria, the adhesive composition may still prove undesirable from an aesthetic and practical point of view. Aesthetically, the adhesive composition utilized must be substantially white or clear in color. Otherwise, the stripes of adhesive will be particularly noticeable and unsightly in the disposable article which is generally composed of clear and white components.
On a practical level, the adhesive must possess resistance to heat degradation, oxidation and skin formation during the construction phase of these disposable articles. During this phase, the hot melt adhesives are sometimes held molten and exposed to air for several days at temperatures near 175.degree. C. in the adhesive application equipment. Additionally, the adhesive may be remelted one or more times during this period of days. Adhesive stability in this very severe environment is of major concern, especially if the adhesive contains unsaturated polymer components which tend to be thermally unstable.
An indication of degradation is skin formation on the surface of the adhesive. Such skin formation poses a serious problem to the adhesive application equipment. Specifically, during agitation, the skin tends to fragment and flake into the molten adhesive. These flakes of skin, thereafter, tend to clog the nozzles which finally apply the adhesive onto the disposable article. At a minimum, the clogging may produce articles which do not meet specifications, but eventually call for a maintenance shutdown of the manufacturing line to unclog these nozzles.
As briefly alluded to above, polymers form a substantial portion of the adhesive composition. Particularly, block copolymers have been increasingly employed in adhesive compositions primarily due to their high cohesive strengths and their ability to "crosslink" without a chemical vulcanization step. These block copolymers, such as described in Harlan, U.S. Pat. No. 3,239,478, are primarily either linear or radial styrene/butadiene or styrene/isoprene block copolymers. The linear copolymers typically have the structure styrene-butadiene-styrene (SBS) or styrene-isoprene-styrene (SIS). The high cohesive strength of these styrene/diene block copolymers primarily results from their well-known domain formation.
However, the polymerization process utilized for forming these block copolymers commonly employs a brominated polyfunctional coupling agent, which produces a brominated residue. These residues are suspected of causing a color instability in the adhesive composition during molten aging. The color instability manifests itself as a developing and thereafter darkening brown color with the passage of time, thereby turning a clear or hazy white adhesive to a translucent dark brown color.
It has now been found that a particular combination of particular antioxidants, stabilizers and halide scavengers can be be employed in adhesive compositions containing block copolymers formed using brominated polyfunctional coupling agents and results in a degradation resistant, color stable adhesive composition useful in multi-line construction of disposable articles such as diapers, sanitary napkins and bed pads.